Voltage artifacts, which interfere with the ECG signal presented to the ECG monitor, are common when using standard diagnostic type ECG equipment in the MRI environment. These interfering signals are induced in the patient cable and the electrode leads which connect the ECG electrodes on the patient inside the MRI machine to the ECG monitor and sequence trigger system. Two sources of these interfering signals are the radio frequency (RF) pulses and the magnetic field gradient pulses which are generated by the MRI machine. In the case of the RF pulses, they not only cause a degradation in the cardiac signal measurement, but they often set up very large electrical currents in the connecting cables and leads. These cables and leads can become very hot and burn the patient if they are in contact with the patient's skin.
A third source of voltage artifacts, known as motion artifacts, also exists. These interfering signals are picked up by induction in the connecting leads or directly from the patient through the ECG electrodes. One example of a motion artifact is caused by patient breathing. The electrode leads, which are attached to the electrodes on the patient's chest, move in the primary magnetic field of the MRI magnet. Voltages are induced in the leads as a result of lead motion and are combined with the cardiac voltage at the input to the monitor. Another example of a motion artifact is one caused by patient blood flow. Rapid blood flow in the aorta and other large heart vessels also generates interfering signals. This is due to the blood being an electrical conductor that is moving in a magnetic field. This artifact, combines with the cardiac voltage within the patient and, is picked up on the skin by the ECG electrodes.
Conversely, certain artifacts in the MRI image can result from high frequency signals generated by ECG electronics. These signals are radiated as electromagnetic waves to the MRI machine through the patient cable and the electrode leads which act as transmitting antennas. These artifacts degrade the MRI image. Placement of the electronics can also effect the MRI imaging by obscuring a particular area.
A device for monitoring ECG's and respiration in an MRI setting, while minimizing artifacts is described in U.S. Pat. No. 4,763,075. An electronic sensor module, including an RF filter, amplifier stage and slew rate filter, which filters RF and interfering pulses from the ECG and respiration signals is located outside the MRI tunnel. This filtered signal is transmitted to a patient monitor located outside of the MRI shielded room through optic cables to avoid any further interference by the MRI. One disadvantage of the system described in U.S. Pat. No. 4,763,075 is the need to use long cables and leads because the electronics sensor module is located outside of the MRI tunnel. This can result in a very large electrical current being generated in the long cables and leads, which can become very hot and burn the patient. In addition, the long cables can generate a field which can also interfere with the MRI image. Another disadvantage of this device is the difficulty of monitoring an unstable patient since the ECG monitor must be outside of the MRI shielded room. A further disadvantage of this system is the inability to control the selection of the active leads of the ECG, and the inability to monitor a "lead-off" condition.
Another device for monitoring ECG's in an MRI setting, while attempting to minimize artifacts is described in French Patent National Registration No. 930491. The electronic sensor module of the device described in this reference is designed to be used within the MRI tunnel. However, it includes electrodes rigidly attached to the module which is strapped directly onto the patient. A disadvantage of this device is that the entire module must be placed close to the heart for monitoring the patient's ECG. This arrangement can obscure the heart and other areas from MRI imaging and has little effect on minimizing motion artifacts, specifically those artifacts due to the blood flow. In addition, there is no remote control of the active leads of the ECG. Furthermore, this device does not include a slew-rate filter for suppressing voltage spikes and gradient pulses picked up by the electrode leads. In addition, no provision is made for monitoring a lead-off condition or the battery charge state of the device.
Other devices for monitoring ECG's in an MRI setting, while attempting to minimize artifacts are described in U.S. Pat. Nos. 4,694,837 and 5,038,785. The electronic sensor modules described in these references are designed to be used within the MRI tunnel. However, there is no remote control of the active leads of the ECG. In addition, there is no provision for monitoring a lead off condition or the battery charge state of the device.
It is therefore an objective of the present invention to provide a system for reducing voltage artifacts during the making of ECG's while using MRI's.
It is another objective of the present invention to provide a system for reducing motion artifacts during the making of ECG's while using MRI's.
It is yet a further objective of the present invention to allow remote control and monitoring of the system during the making of ECG's to allow for the best output signal.
It is yet another objective of the present invention to provide a system that can be moved to prevent the obscuring of body parts from MRI imaging.